Capacitor cells, such as batteries and capacitors, are particularly useful in various implantable medical devices, such as implantable defibrillators. Implantable defibrillators are implanted into the chests of patients prone to suffering ventricular fibrillation, a potentially fatal heart condition. Batteries and capacitors are critical components in these devices because they store and deliver one or more life-saving bursts of electric charge to a fibrillating heart. Specifically, lithium batteries and electrolytic capacitors are commonly used in implantable defibrillators. One drawback with current batteries and capacitors is that they are often large in size, which in turn causes the total size of the defibrillator to be relatively large in size. A patient who has such a device implanted may be bothered by the presence of such a large object in the chest area. Thus, it would be desirable to produce a smaller and more compact capacitor cells for use in implantable medical devices.
One important component in capacitor cells is the separator. Capacitor cells typically comprise an anode, a cathode, a separator, and an electrolyte solution. The anode serves as the positive source of energy and the cathode serves as the negative source of energy. The separator is a nonconductive dielectric that serves to keep the anode and cathode separate from one another. Separation of the anode and cathode is necessary to prevent short circuiting. The electrolyte is an ionic solution that interacts with the anode to form a dielectric oxide layer thereon. The dielectric oxide layer serves to insulate the anode from the surrounding electrolyte solution, allowing for charge to accumulate. The energy of the capacitor cell is stored in the electromagnetic field generated by the opposing electrical charges separated by the dielectric layer disposed on the surface of the anode.
Separators are generally made from a roll or sheet of separator material, and a variety of separator materials have been found to be effective. Paper, particularly Kraft paper, is a cellulose-based separator material that is commonly used. Cellulose separator materials are manufactured with high chemical purity. Metalized paper and paper & foil constructions are useful variants of cellulose-based separators.
A common alternative to paper separators are polymeric separators. Generally, polymeric separators are either made of microporous films or polymeric fabric. An example of a microporous film separator is a separator comprising polytetrafluoroethylene, disclosed in U.S. Pat. No. 3,661,645 to Strier et al. U.S. Pat. No. 5,415,959 to Pyszeczek et al., on the other hand, describes the use of woven synthetic halogenated polymers as capacitor separators. The use of “hybrid” separators comprising polymer and paper material has also been described. See, for example, U.S. Pat. No. 4,480,290 to Constanti et al., which describes the use of separators including a porous polymer film made from polypropylene or polyester and absorbent paper.
While paper and polymeric separators have been satisfying in use, several drawbacks still remain. One drawback is that current separator materials are often very thick. The use of thick separator materials makes it difficult to manufacture capacitors having decreased sizes. For example, the total thickness of cellulose separators employed between anode and cathode plates will vary with the voltage rating of the capacitor structure and the type of electrolyte employed, but on the average, the thickness varies from 0.003 inches to 0.008 inches in connection with capacitors rated at from 6 volts to 600 volts, respectively. If the thickness of the separator material can be decreased, the diameter and volume of the capacitor elements can be decreased, which would reduce the space necessary to contain the capacitors. Thus, it would be desirable to employ a thinner separator material to reduce the space necessary to contain a capacitor.
A further difficulty with prior separators is that they do not have strong enough tensile properties. A separator with strong tensile properties is desirable because it is less likely to tear or break during fabrication of the capacitor cell. Strong tensile properties are also desirable because they are more able to withstand internal stresses in the capacitor cell due to changes in the anode and cathode volumes during discharge and re-charging cycles. Single sheets of paper separator materials alone do not have strong tensile properties. Rather, the paper separators are often made thick in order to increase its tensile properties. However, thick paper separators are undesirable, as more space is needed to contain the capacitor. Likewise, microporous polymeric films can also be made very thin which contributes to volumetric efficiency of the capacitor, but this reduction is thickness is accompanied by a reduction in strength.
Another drawback with prior separators is that separator materials often unpredictably swell or expand when impregnated with liquid electrolyte. This swelling often causes the remaining capacitor elements to be pushed outward, which often results in swelling of the capacitor enclosure. The swelling may also cause damage to capacitor elements, rendering the capacitor dysfunctional. In order to account for this swelling, a larger-sized capacitor enclosure is sometimes used, again causing an undesirable increase in the amount of space needed to contain the capacitor. Thus, it would be desirable to employ a separator material that is resistant to swelling when impregnated with electrolyte.
A yet another drawback with prior separators is that porosity is not precisely controlled. Typically, a separator material should have a porosity small enough to maintain enough separation so that the electrical resistivity is sufficiently high to prohibit short circuit current from flowing directly between the anode and cathode. The inventors have discovered that the tortuosity of the separator material also affects ionic transport through the working electrolyte; for example, a separator material having on average relatively non-tortuous paths through the separator appears to possess superior performance as compared to a high tortuosity separator material. Likewise, the porosity should be high enough to allow the ions within the electrolyte to be transferred through the separator material. The ideal porosity of a given separator material depends on the specific capacitor and components used thereinwith. While porous polymers are currently used as separator materials, it has often been difficult to precisely control their porosity so that separator performance can be optimized. Factors influencing porosity include pore size, shape, density, and distribution. Thus, it would be desirable to employ a separator material that has a precise porosity that would be ideal for used with a particular capacitor cell.
In addition to overcoming the above drawbacks, an ideal separator material will also be resistant to degradation in the cell environment and exhibit surface energy such that electrolyte wettability and absorption are augmented. Therefore, it would be desirable to employ in electrolytic capacitors and other capacitor cells a separator material that is sufficiently thin, has strong tensile properties, is resistant to swelling, is resistant to degradation in the cell environment, and has a precise porosity sufficient for use with a particular capacitor cell.